It's an insurance program. When we contribute to the Canada pension plan, we also automatically contribute to this insurance fund. The program is set up to provide these insurance funds to people who meet a minimum number of criteria. For example, a person needs to have previously contributed to the program, and so on. Once you are eligible for the program, there is a basic benefit amount, a minimum but still respectable amount, that everyone gets. This way, even if you haven't been contributing to the program for very long, you still get a minimum amount. Added to that is a percentage of what you would have received for a pension, for example. It's the combination of those two amounts.
It's a little like the Canada pension plan, which was not put in place with the intent of replacing 100% of a person's employment income. It was designed to help and replace up to 25% of employment income.
When a person benefits from the disability program, amounts are therefore added. Other programs, including tax credits, are also in place to help these people.